UNDERWOOD, Iowa — Vermont Sen. Bernie Sanders sought to chip away at one of rival Hillary Clinton's chief arguments against him Tuesday, saying that the enthusiasm surrounding his White House bid puts him in the strongest position to increase voter turnout and defeat a Republican in the November election.

During a four-stop swing through Iowa, Sanders also predicted a "very close" race with Clinton in the state's lead-off caucus on Feb. 1.

"Today the inevitable candidate doesn't look quite so inevitable as she did 8 1/2 months ago," Sanders said during a town hall meeting on a frigid morning in Fort Dodge. He later told reporters that if he were in Clinton's position, "I would be nervous."

Clinton's campaign, seeking to blunt Sanders' momentum, refuted the senator's electability assertion. The campaign pointed to recent supportive comments about Sanders from Republican Party leaders as an indication that the GOP believes he would be easier to defeat in November than Clinton.

"While Sen. Sanders tries to make a case on electability based on meaningless polls, Republicans and their super PACs have made clear the candidate they're actually afraid to face," said Jennifer Palmieri, Clinton's communications director.

It was the latest volley in a Democratic race that has become increasingly heated in recent days as Sanders challenges Clinton's lead in Iowa. He also appears to have an edge over Clinton in New Hampshire, which votes second.

Clinton's campaign also challenged the senator's foreign policy credentials Tuesday, particularly regarding Iran and the Islamic State group. The campaign released a statement Tuesday from 10 former diplomats and national security officials who said they were concerned Sanders "has not thought through these crucial national security issues that can have profound consequences."

Demonstrators protest in front of supporters of Hillary Clinton as they rally outside the Gaillard Center before the start of the the Jan. 17 Democratic debate in Charleston, S.C.
Scott Olson, Getty Images

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Sanders rarely mentions foreign policy in his campaign speeches, which are focused almost entirely on economic issues and other domestic policy matters. He's acknowledged that Clinton, a former secretary of state, has more experience on foreign policy matters, but he has held up her vote for the 2003 invasion of Iraq as a significant error in judgment.

"On the crucial foreign policy issue of our time, it turns out that Secretary Clinton, with all of her experience, was wrong and I was right," he said Tuesday.

Clinton's overarching argument against Sanders is that nominating the self-declared democratic socialist would put the party at risk of losing to Republicans in November. Clinton has also accused the senator of flip-flopping on guns and called his health care proposal expensive and unrealistic.

Asked by a voter how he would get a divided Congress to rally behind his plans, Sanders said it would depend on Americans "standing up saying enough is enough."

Still, Sanders — who has served in Congress since 1991 — said he opposed term limits on lawmakers, calling it "an easy way out of a complicated problem."

Sanders also renewed his call for breaking up big Wall Street banks, one of the central tenets of his campaign, and his criticism of Clinton for accepting massive speaking fees from Goldman Sachs.

After railing against the bank's recent $5 billion settlement for selling faulty mortgage securities in the lead-up to the 2008 financial crisis, Sanders said, "Goldman Sachs also provides, very, very generous speaking fees to some unnamed candidates"

The senator was traveling across Iowa Tuesday in a new custom campaign bus that won rave reviews from the candidate.

"You can walk around, it's got comfortable seats," he told voters at a town hall in Carroll, Iowa.